I would use my backhoe and dig out a pit in the four foot range and go down about 8 feet if it was me. I have a backhoe, so it works for me. I would do this many times, all over the area you plan on putting the pond. Look at the dirt that you dig up and see what it's made of.
If you have a post hole digger, go down as far as you can and do it all over the place.
Then dump water in the hole and watch it. A five gallon bucket will take quite a few trips to get enough water in the hole, but at least you will find out if it's holding water. Either way, it's important information.
Eddie
